Technical Abstract:
Thermal infrared (TIR) remote sensing of land-surface temperature (LST) provides valuable information about the sub-surface moisture status: soil surface temperature increases with decreasing water content, while moisture depletion in the plant root zone leads to stomatal closure, reduced transpiration, and elevated canopy temperatures. In this paper, a satellite-based methodology for routine drought monitoring will be described using basin- to continental-scale maps of evapotranspiration (ET) obtained with a TIR-based surface energy balance model. In this approach, moisture stress is quantified in terms of the reduction of ET from the potential rate (PET) expected under non-moisture limiting conditions.
The Atmosphere-Land Exchange Inverse (ALEXI) model is used to map land-surface water and energy fluxes across the continental U.S. at 100m to 10km resolution using TIR imagery from polar orbiting and geostationary satellites. A derived Evaporative Stress Index (ESI), describing standardized anomalies in the ET/PET ratio, shows good correspondence with standard drought metrics and with patterns of antecedent precipitation, but at significantly higher spatial resolution due to limited reliance on ground observations. The ALEXI ESI algorithm does not require precipitation or soil texture information, unlike the Palmer Drought Index, the Standardized Precipitation Index, and other drought indices based on rainfall or soil water balance. Being an independent means for assessing drought conditions, the ESI has significant potential for enhancing the existing suite of drought monitoring products. Work is underway to further evaluate multi-scale ESI implementations over the U.S. and other continents with geostationary satellite coverage.